Continuous casting of metals such as aluminum alloys has been performed in continuous casters, such as twin roll casters and belt casters. Twin roll casting traditionally is a combined solidification and deformation technique involving feeding molten metal into the bite between a pair of counter-rotating cooled rolls wherein solidification is initiated when the molten metal contacts the rolls. Solidified metal forms as a “freeze front” of the molten metal within the roll bite and solid metal advances towards the nip, the point of minimum clearance between the rolls. The solid metal passes through the nip as a solid sheet. The solid sheet is deformed by the rolls (hot rolled) and exits the rolls. Belt casting generally involves delivering molten metal to a pair of endless belts each moving over an entry pulley and an exit pulley. The metal solidifies between the belts during the time that the belt travels from the entry pulleys to the exit pulleys.
Aluminum alloys have successfully been twin roll cast into about ¼ inch thick sheet at about 4–6 feet per minute or about 50–70 pounds per hour per inch of cast width (lbs/hr/in). Attempts to increase the speed of twin roll casting typically fail due to centerline segregation. Although it is generally accepted that reduced gauge sheet (e.g. less than about ¼ inch thick) potentially could be produced more quickly than higher (thicker) gauge sheet in a twin roll caster, the ability to twin roll cast aluminum at rates significantly above about 70 lbs/hr/in has been elusive.
Typical operation of a twin roll caster at thin gauges is described in U.S. Pat. No. 5,518,064 (incorporated herein by reference) and depicted in FIGS. 1 and 2. Molten metal M is supplied via a tip T to a pair of water-cooled twin rolls R1 and R2 rotating in the direction of the arrows A1 and A2, respectively. The centerlines of the rolls R1 and R2 are in a vertical or generally vertical plane L (e.g. up to about 15° from vertical) such that the cast strip S forms in a generally horizontal path. Other versions of this method produce strip in a vertical direction. The width of the cast strip S is determined by the width of the tip T. The plane L passes through a region of minimum clearance between the rolls R1 and R2 referred to as the roll nip N. A solidification region exists between the solid cast strip S and the molten metal M and includes a mixed liquid-solid phase region X. A freeze front F is defined between the region X and the cast strip S as a line of complete solidification.
In conventional roll casting, the heat of the molten metal M is transferred to surfaces U1 and U2 of the rolls R1 and R2 such that the location of the freeze front F is maintained upstream of the nip N. In this manner, the molten metal M solidifies at a thickness greater than the dimension of the nip N. The solid cast strip S is deformed by the rolls R1 and R2 to achieve the final strip thickness. Hot rolling of the solidified strip between the rolls R1 and R2 according to conventional roll casting produces unique properties in the strip characteristic of twin roll cast metal strip. For an aluminum alloy, a central zone through the thickness of the strip becomes enriched in esthetic forming elements (esthetic formers) in the alloy such as Fe, Si, Ni, Zn and the like and depleted in peritectic forming elements (Ti, Cr, V and Zr). This enrichment of esthetic formers (i.e. alloying elements other than Ti, Cr, V and Zr) in the central zone occurs because that portion of the strip S corresponds to a region of the freeze front F where solidification occurs last and is known as “centerline segregation”. Extensive centerline segregation in the as-cast strip is a factor that restricts the speed of conventional twin roll casters. The as-cast strip also shows signs of working by the rolls. Grains which form during solidification of the metal upstream of the nip become flattened by the rolls. Therefore, roll cast aluminum includes grains with multiaxial (non-equiaxed) structure.
The roll gap at the nip N may be reduced in order to produce thinner gauge strip S. However, as the roll gap is reduced, the roll separating force generated by the solid metal between the rolls R1 and R2 increases. The amount of roll separating force is affected by the location of the freeze front F in relation to the roll nip N. As the roll gap is reduced, the percentage reduction of the metal sheet is increased, and the roll separating force increases. At some point, the relative positions of the rolls R1 and R2 to achieve the desired roll gap cannot overcome the roll separating force, and the minimum gauge thickness has been reached for that position of the freeze front F.
The roll separating force may be reduced by increasing the speed of the rolls in order to move the freeze front F downstream towards the nip N. When the freeze front F is moved downstream (towards the nip N), the roll gap may be reduced. This movement of the freeze front F decreases the ratio between the thickness of the strip at the initial point of solidification and the roll gap at the nip N, thus decreasing the roll separating force as proportionally less solidified metal is compressed and hot rolled. In this manner, as the position of the freeze front F moves towards the nip N, a proportionally greater amount of metal is solidified and then hot rolled at thinner gauges. According to conventional practice, roll casting of thin gauge strip is accomplished by first roll casting a relatively high gauge strip, decreasing the gauge until a maximum roll separating force is reached, advancing the freeze front to lower the roll separating force (by increasing the roll speed) and further decreasing the gauge until the maximum roll separating force is again reached, and repeating the process of advancing the freeze front and decreasing the gauge in an iterative manner until the desired thin gauge is achieved. For example, a 10 millimeter strip S may be rolled and the thickness may be reduced until the roll separating force becomes excessive (e.g. at 6 millimeters) necessitating a roll speed increase.
This process of increasing the roll speed can only be practiced until the freeze front F reaches a predetermined downstream position. Conventional practice dictates that the freeze front F not progress forward into the roll nip N to ensure that solid strip is rolled at the nip N. It has been generally accepted that rolling of a solid strip at the nip N is needed to prevent failure of the cast metal strip S being hot rolled and to provide sufficient tensile strength in the exiting strip S to withstand the pulling force of a downstream winder, pinch rolls or the like. Consequently, the roll separating force of a conventionally operated twin roll caster in which a solid strip of aluminum alloy is hot rolled at the nip N is on the order of several tons per inch of width. Although some reduction in gauge is possible, operation at such high roll separating forces to ensure deformation of the strip at the nip N makes further reduction of the strip gauge very difficult. The speed of a roll caster is restricted by the need to maintain the freeze front F upstream of the nip N and prevent centerline segregation. Hence, the roll casting speed for aluminum alloys has been relatively low.
Continuous casting of aluminum alloys has been achieved on twin belt casters at rates of about 20–25 feet per minute at about ¾ inch (19 mm) gauge reaching a productivity level of about 1400 pounds per hour per inch of width. An example of conventional belt casting is described in U.S. Pat. No. 4,002,197. In twin belt casting, molten metal is fed into a casting region between a pair of moving belts that each revolve around a pair of pulleys. The metal solidifies as it is carried along between the belts and the heat is liberated from the solidifying metal by cooling the inside surfaces of the belts with rapidly moving films of liquid (e.g. water) traveling along the inside surfaces.
The operating parameters for belt casting are significantly different from those for roll casting. In particular, there is no intentional hot rolling of the strip. Solidification of the metal is completed in a distance of about 12–15 inches (30–38 mm) downstream of the nip for a thickness of ¾ inch. The belts are exposed to high temperatures when contacted by molten metal on one surface and are cooled by water on the other surface. This temperature differential may lead to distortion of the belts. The tension in the belt must be adjusted to account for expansion or contraction of the belt due to temperature fluctuations in order to achieve consistent surface quality of the strip. Casting of aluminum alloys on belt casters has been used to date mainly for products having minimal surface quality requirements, such as products which are subsequently painted.
In part of efforts to improve surface quality of belt cast strip, improved heat transfer from the molten metal to a casting surface has been attempted in certain modified belt casters as described in U.S. Pat. Nos. 5,515,908 and 5,564,491 shown schematically in FIGS. 3 and 4. A belt caster generally includes a pair of endless belts B carried by a pair of upper pulleys U and a corresponding pair of lower pulleys P. The arrangement of the pulleys U and P one above the other defines a molding zone Z bounded by the belts B. The gap between the belts B determines the thickness of the strip S, with the gap being most narrow at the nip N between the entry pulleys along the vertical plane L. Molten metal M fed directly via a trough R and tip T into the nip N is confined between the moving belts B and is solidified as it is carried along. Heat liberated by the solidifying metal is withdrawn through the portions of the belts B which are adjacent to the metal being cast. This heat may be withdrawn by cooling the reverse surfaces of the belts via cooling means C such as nozzles positioned to spray a cooling fluid onto the reverse surfaces of the belts or by employing exit pulleys having circumferential channels containing cooling fluid that contacts the belt reverse surfaces as described in U.S. Pat. No. 6,135,199. In a heat sink belt caster, molten metal is delivered to the belts (the casting surface) upstream of the nip with solidification initiating prior to the nip and continued heat transfer from the metal to the belts downstream of the nip. In this system, molten metal is supplied to the belts along the curve of the upstream rollers so that the metal is substantially solidified by the time it reaches the nip between the upstream rollers. The heat of the molten metal and the cast strip is transferred to the belts within the casting region (including downstream of the nip). The heat is then removed from the belts while the belts are out of contact with either of the molten metal or the cast strip. In this manner, the portions of the belts within the casting region (in contact with the molten metal and cast strip) are not subjected to large variations in temperature as occurs in conventional belt casters. The thickness of the strip is limited at least in part by the heat capacity of the belts between which casting takes place. Production rates of up to 2400 lbs/hr/in for 0.08–0.1 inch (2–2.5 mm) strip have been achieved.
However, problems associated with the belts used in conventional belt casting remain. In particular, dimensional uniformity of the cast strip depends on the stability of (i.e. tension in) the belts. For any belt caster, conventional or heat sink type, contact of hot molten metal with the belts and the heat transfer from the solidifying metal to the belts creates instability in the belts. Under certain conditions, the belts that are in contact with the recently solidified strip can cause the strip edges to peel away.
Accordingly, a need remains for a method of high-speed continuous casting of aluminum alloys which minimizes the contact of belts with solidifying metal yet achieves uniformity in the cast strip surface at high production rates.